1. Field of the Invention
This invention relates generally to resonant sensing devices and methods which can be used to determine or test the physical properties or characteristics of a test piece or solid particularly at the surface of the latter.
2. Description of the Prior Art
It is known, for example, as disclosed in my U.S. Pat. No. 3,153,338, issued Oct. 20, 1964, to provide a resonant sensing device for indicating the surface properties, for example, hardness, of a test piece. Such known device comprises a mechanical resonating means or sensor having a hemispherical, conical, pyramidal or otherwise shaped contact surface to afford contact with a test piece over progressively increasing areas with increasing penetration or indentation of a surface of the test piece by the contact surface of the mechanical resonating means, electrically energized means for effecting vibration of the mechanical resonating means at a resonance frequency of the latter, and means exerting a static force for holding the contact surface of the vibrated mechanical resonating means in steady contact with the test piece so as to cause plastic and/or elastic indentation or deformation of the surface of the latter to an extent dependent upon the surface characteristics of the test piece and the magnitude of the static force. Two basic methods of operating the above described device have been proposed.
In one of these methods of operation, a constant static force is used for maintaining the steady contact and a tunable generator is used for effecting vibration of the resonating means. When the resonance frequency of the mechanical resonating means is altered from the resonance frequency in its free state by reason of its steady contact with the test piece, the generator frequency is varied until resonance is restored, and such variation of the generator frequency is measured as an indication of the surface characteristics, for example, the hardness of the test piece.
In another method of operating the described device, the frequency at which the mechanical resonating means is vibrated is fixed at a value different from the resonance frequency of the mechanical resonating means when in the free state or condition, and the static force by which the mechanical resonating means is held in steady contact with the test piece is progressively increased until resonance is achieved. In this method of operation, the magnitude of the static contact force required to produce resonance is measured as an indication of the surface characteristics of the test piece.
It has further been known, for example, as disclosed in my U.S. Pat. No. 3,572,097, issued Mar. 23, 1971, to operate a resonant sensing device, as aforesaid, with both the generator frequency and the static contact force being fixed, and with any variation of the resonance frequency of the mechanical resonating means resulting from its contact with the test piece being cancelled or reversed by varying the free resonance frequency of the resonating means, that is, by varying a characteristic or parameter of the mechanical resonating means which determines its resonance frequency, whereupon, such variation of the characteristic or parameter is measured as an indication of the surface properties of the test piece. Among the resonance frequency determining characteristics or parameters of the mechanical resonating means that may be varied are an elastic modulus thereof, as by varying a magnetic polarizing field in the case where the mechanical resonating means includes a magnetostrictive rod or an electric polarizing field in the case where the mechanical resonating means includes an electrostrictive or piezo-electric rod, or by varying the temperature of the mechanical resonating means. Further, where the mechanical resonating means includes an electromechanical transducer, its varied characteristic or parameter may be the coupling thereof to electrical circuits by which the transducer is energized from the fixed or constant frequency generator. The varied characteristic or parameter of the mechanical resonating means which determines its resonance frequency may also be its shape, mass distribution relative to its nodes and loops of vibration, or a force other than the static contact force which is applied to the mechanical resonating means remote from its contact surface to add elastic strain energy.
Although some of the above described resonant sensing devices and methods of operating the same have enjoyed widespread commercial success, it has been found that, under certain conditions, appreciable errors arise in the measurements of the hardness or other surface properties obtained thereby. More particularly, in the theoretical basis for the existing resonant sensing devices, it has been assumed that the mechanical reactance of the test piece is infinitely large or at least very much larger than the contact reactance. This assumption is reasonable in many cases, for example, when the test piece is relatively large and/or massive or, in the case of a relatively small test piece, when such test piece can be adequately clamped or mounted, for example, by grease-coupling of the test piece to a heavy base such as a steel block, so that the resulting measuring error is then within acceptable limits. However, when flexural waves are excited in the test piece, as may happen when the test piece is very small or thin or is thin-walled and hollow and thus cannot be supported at the surface opposed to that engaged by the contact surface of the resonating means, the error which results from the reactance of the test piece may not be negligible. A similar problem may arise, even with a test piece of substantial size or mass, if a large static force, for example, of the order of 100 kg., is employed for holding the contact surface of the vibrated mechanical resonating means in steady contact with the test piece.